''We use FEA
to design parts of the vehicle to be strong enough to
perform their intended functions,'' says Joe Partain,
Director of Design at Zeus. FEA refers to ''finite element
analysis,'' a valuable practice whose purpose is to numerically
calculate stresses in a vehicle part under a set of loading
and boundary conditions.
''It's also used to
lighten a part. It's a tool used to identify low-stress
areas, indicating where you can reduce the amount of
material or remove sections altogether. It also indicates
high-stress areas in the design, so you can reinforce
those places.''
Using COSMOS, an industry standard FEA
software package, design engineers are able to not only
create nonlinear material models but also simulate mechanical
events for testing. Digital simulations provide a cost-effective
way to run batches of tests on vehicle systems designs
without the costly setup necessary for real-life testing.
''You can choose to do a linear stress
analysis, where the stress in the part under analysis
is valid as long as the stress is below the yield point
of the material,'' Partain remarks, explaining the distinction
between linear and nonlinear models. The point when material
starts to yield marks the entry into the nonlinear material
range.
In a specific example, Zeus experienced
some deflection problems with the swing arms in a suspension
system in an early prototype. |
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''Through FEA, we were able to increase the strength
of the swing arm without increasing the weight,'' Partain informs.
Analysis and testing led to the discovery of a simple solution
that adjusted the thickness and shape of the material in
the final swing arm design.
In the beginning of the design process, Zeus's design engineers
engage heavily in vehicle dynamics analysis: Define the
roll axis. What's the motion ratio of the wheel to the shock?
What's the camber change? How much bump steer is induced
during suspension travel at full turn as well as under straight
driving conditions?
Then, as the process progresses and parts start maturing
in their designs, the engineers begin using FEA to optimize
and verify stress levels. ''We load a part to see if we have
enough material in localized areas and to see whether the
part has a propensity to break,'' shares Design Engineer
Hayden Barr.
After optimizing the components within their various systems,
the engineers simulate standard mechanical events to test
performance.
''A good test to begin with is a two-foot-high bump,'' says
Partain. A two-foot bump is ideal because it represents a
worst-case scenario because as the front tires climb up the
bump and then leave the top of the bump into the air, the
rear tires contact the bump, producing a pitching motion.
'We simulate the pitch of the vehicle as it goes over the
bump at various speeds,'' he adds.
Another benchmark test involves a series of sine wave bumps,
which occurs naturally in areas where there is frequent usage
of ATVs or off-road vehicles. A sinusoidal set of bumps develops
over time in these high-traffic areas, induced by each vehicle's
natural frequency.
''We can recreate a similar set of bumps analytically in
the model. If we move our vehicle over the bumps at a speed
near the natural frequency of the suspension, the vehicle
begins to pitch,'' Partain asserts.
By incorporating in the design proper shock absorbers, damping
rates in compression and rebound and spring rates in the
front and rear, the engineers can minimize the occurrence
of pitching and prevent the likelihood of a nose-first roll.
The engineers also run simulations on the suspension corner
units to model behavior over severe broken ground.
''There,'' declares Partain, ''we're monitoring forces in
the suspension, forces in the shock absorber, accelerations
to the occupant, drivability and feedback into the steering
mechanism.''
Furthermore, Zeus simulates the vehicle mounting and leaving
an angled set of bumps designed to twist the frame. This
simulation demonstrates the torsional stiffness of the chassis.
Strategic use of FEA software during the design process
results in individual components with a great strength-to-weight
ratio, as well as safe, high-performance vehicles.
In compact, lightweight off-road vehicle
design, function is of the essence, so each part as well
as the complete vehicle must contain adequate materials
to achieve their design purposes. Nothing more, and nothing
less.
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